Electric devices are often provided with air-tight structures to prevent internal components from being affected by moisture and other foreign matter. Such devices therefore need to be tested after assembly if they are indeed air-tight, typically by introducing pressurized air into the interior of the device, and then detecting any drop in the internal pressure over time.
FIG. 3 shows an electric power steering (EPS) device 20 of a rack and pinion type for automotive use which is given as an example of the electric device whose air-tightness is desired to be tested. This EPS device 20 comprises a motor housing 21a, a gear housing 21b, a rack shaft 23 extending axially through the motor housing 21a and the gear housing 21b in an axially slidable manner, and a hollow motor rotor 22 which is rotatably fitted onto the rack shaft 23 in the motor housing 21a and engages the rack shaft 23 via a ball screw mechanism not shown in the drawing. Thus, the output torque of the motor rotor 22, which is received in the motor housing 21a, is converted into the axial thrust of the rack shaft 23. In the gear housing 21b, a pinion attached to a steering shaft 24 meshes with a rack of the rack shaft 23 so that the steering effort applied to the steering wheel, which is attached to the steering shaft 24, is assisted by the thrust of the rack shaft 23 produced by the motor rotor 22.
When testing this EPS device 20 for its air-tightness, conventionally, pressurized air is introduced into the interior of the EPS device 20 from a rack guide mounting hole 25, which is provided in a side wall of the gear housing 21b originally for the purpose of fitting a rack guide (not shown in the drawing) therein along with a compression coil spring (not shown in the drawing). The rack guide applies a biasing spring force to the rack shaft 23 to control the force with which the pinion of the steering shaft 24 meshes with the rack, and this spring force can be adjusted by a screw 26 threadably received in the rack guide mounting hole 25.
In the assembled state of the EPS device 20, the rack guide mounting hole 25 is closed off by the screw 26 with the aid of an O-ring fitted in an annular groove formed around the screw 26. When conducting an air-tightness test, after the screw 26 is removed from the rack guide mounting hole 25 or when the EPS device 20 is fully assembled with the exception of the screw 26, a testing head (not shown in the drawing) is fitted into the rack guide mounting hole 25. Pressurized air is introduced into the housing of the EPS device 20 from the testing head. When a prescribed pressure is achieved in the housing of the EPS device 20, the supply of the pressurized air is discontinued, and the internal pressure of the EPS device 20 is measured if there is any drop in the internal pressure of the EPS device housing. Upon completion of the test, the testing head is removed, and the screw 26 is threaded into the rack guide mounting hole 25. Therefore, even when favorable test results are obtained, if the screw 26 is not properly threaded into the rack guide mounting hole 25, or the O-ring is dislodged from the annular recess formed around the screw 26, the housing of the EPS device 20 would not be air-tight in spite of satisfactory test results. This can be prevented by using a relatively large O-ring, and/or by using utmost care when threading the screw 26 into the rack guide mounting hole 25, but it will increase the cost for producing the EPS device.